Gate valves are typically used for controlling a fluid flow in pipe systems, such as water supply or sewer systems. A double-disc gate valve typically comprises a casing forming a fluid passageway between two valve ports and two parallel valve discs movable in an inwards direction into the passageway and in an outwards direction out of the passageway.
A fluid conduit, e.g. a water pipe, is connected to each valve port. Typically, the valve ports are arranged so that the two pipes are axially aligned. Each valve port is surrounded by a valve seat inside the casing.
In a closed configuration of the valve, each valve disc along its periphery abuts a respective one of the valve seats, thereby fluidly separating the corresponding pipe from the inside of the valve casing and thus blocking the fluid flow. To enter an open configuration of the valve, the valve discs are moved outwardly. In practice, the discs are retracted substantially radially relative to the passageway and thereby perpendicularly to the flow direction. In this process, the discs move into a resting compartment of the casing which is arranged radially outside the passageway. To reenter the closed configuration of the valve, the valve discs are moved in the opposite direction, inwardly from the resting compartment into the passageway.
In fluid systems, the gate valves are typically oriented such that the valve discs move out of the passageway when they move vertically upwards during opening and so that they move into the passageway when moving downwards during closing of the valve. For making reading easier, outwards in the following means out of the passageway, i.e. typically vertically upwards, and inwards in the following means into the passageway, i.e. typically vertically downwards. unless specifically noted otherwise.
To ensure complete fluid separation of the inlet from the outlet of the valve, even at large fluid pressures, the valve typically comprises means for pressing the valve discs apart during the last part of the closing movement, thus forcing the periphery of the valve discs against the valve seats. Furthermore, the casing is typically provided with parallel, vertical tracks or notches, which engage a portion of the periphery of the valve discs or the laterally outer ends of a disc carrier, which holds the valve discs. In this way, the valve discs are guided during the vertical movement into or out of the passageway. The vertical movement of the valve discs is typically effected by a threaded stem or spindle, which may be connected to a handle or a hand wheel for manual operation or to an actuator for e.g. hydraulic or electrical operation.
One important advantage of double-disc gate valves is that they can easily be designed to provide—in the open configuration—a passage through the valve, which allows any object small enough to pass through the pipes to also pass through the valve without restriction. Thus, e.g. inspection, maintenance or cleaning of the piping system may be conducted with remotely operated vehicles (ROV).
Various devices have been invented for urging the valve discs apart during the last part of the closing movement. U.S. Pat. No. 2,502,689 discloses a double-disc gate valve as described above. A threaded stem with a hand wheel is used to move the valve discs vertically. The stem engages an inner thread at the top of the valve casing. Inside the valve casing, the stem is rotatably secured to a partly wedge-shaped disc carrier, which is connected through transversal bolts to lugs on the back side of each valve disc, i.e. the side facing towards the other valve disc. The bolt holes in the lugs are larger than the diameter of the bolts, which are thus allowed to move within the bolt holes. During closing of the valve, the valve discs hang by the lugs on the bolts until the valve discs abut the bottom of the valve casing or are otherwise prevented from moving further downwards. During a further downwards movement of the disc carrier, the wedge-shaped portion thereof slides along inclined surfaces on the back side of each valve disc, thereby urging them apart. During the first part of the opening movement, the bolts will slide along the inclined surfaces on the inside of the bolt holes, thereby pulling the valve discs towards each other to facilitate their subsequent outwards movement into the resting compartment.
FR 2.180.655, JP 63-225768 and JP 5-99348 respectively disclose double-disc gate valves, wherein the valve discs are connected to a disc carrier through a number of linkages, each of which is rotatably connected at one end to a lug on the back side of a valve disc and at the other end to the disc carrier. The linkages are arranged to keep the valve discs parallel at all times. During closing of the valve, the valve discs hang by the linkages on the disc carrier until the valve discs are prevented from moving further downwards, e.g. by the bottom of the casing. During a further downwards movement of the disc carrier, the linkages rotate and urge the valve discs apart. During a first part of the opening movement, the linkages correspondingly pull the valve discs towards each other, while the discs remain parallel.
All of the above described gate valves suffer from the disadvantage that a relatively large force is required to move the valve discs during the last part of the closing movement and during the first part of the opening movement where the discs are moved away from each other or towards each other. With manually operated gate valves this makes it difficult for the operator to close and open the valves. With automatically operated gate valves, the actuators must be dimensioned for this part of the movement, although the remaining part of the movement requires substantially less force. Hence, weight, size and cost of the actuators are relatively high.